The present invention relates generally to the field of ophthalmic surgery and, more particularly, to reducing chatter when carrying out torsional ultrasound while dissipating heat at an incision during phacoemulsification.
The human eye functions to provide vision by transmitting light through a clear outer portion called the cornea, and focusing the image by way of the lens onto the retina. The quality of the focused image depends on many factors including the size and shape of the eye, and the transparency of the cornea and lens.
When age or disease causes the lens to become less transparent, vision deteriorates because of the diminished light that can be transmitted to the retina. This deficiency is medically known as a cataract. An accepted treatment for cataracts is to surgically remove the cataract and replace the lens with an artificial intraocular lens (IOL). In the United States, the majority of cataractous lenses are removed using a surgical technique called phacoemulsification. During this procedure, a thin needle with a distal cutting tip is inserted into the diseased lens and vibrated ultrasonically. The vibrating cutting tip liquefies or emulsifies the lens so that the lens may be aspirated from the eye. The diseased lens, once removed, is replaced by an artificial intraocular lens (IOL).
A typical ultrasonic surgical device suitable for an ophthalmic procedure includes an ultrasonically driven hand piece, an attached cutting tip, an irrigating sleeve and an electronic control console. The hand piece assembly is attached to the control console by an electric cable or connector and flexible tubings. A surgeon controls the amount of ultrasound power that is delivered to the cutting tip of the hand piece and applied to tissue at any given time by pressing a foot pedal to request power up to the maximum amount of power set on the console. Flexible tubings supply irrigation fluid to and draw aspiration fluid from the eye through the hand piece assembly.
The operative part of the hand piece is a centrally located, hollow resonating bar or horn that is attached to a set of piezoelectric crystals. The crystals are controlled by the console and supply ultrasonic vibrations that drive both the horn and the attached cutting tip during phacoemulsification. The crystal/horn assembly is suspended within the hollow body or shell of the hand piece by flexible mountings. The hand piece body terminates in a reduced diameter portion or nose cone at the body's distal end. The nose cone is externally threaded to accept the irrigation sleeve. Likewise, the horn bore is internally threaded at its distal end to receive the external threads of the cutting tip. The irrigation sleeve also has an internally threaded bore that is screwed onto the external threads of the nose cone. The cutting tip is adjusted so that the tip projects only a predetermined amount past the open end of the irrigating sleeve.
In use, the ends of the cutting tip and the irrigating sleeve are inserted into a small incision of predetermined width in the cornea or sclera. One known cutting tip is ultrasonically vibrated along its longitudinal axis within the irrigating sleeve by the crystal-driven ultrasonic horn, thereby emulsifying the selected tissue in situ. The hollow bore of the cutting tip communicates with the bore in the horn that in turn communicates with the aspiration line from the hand piece to the console. Other suitable cutting tips include piezoelectric elements that produce both longitudinal and torsional oscillations. One example of such a cutting tip is described in U.S. Pat. No. 6,402,769 (Boukhny), the contents of which are incorporated herein by reference.
A reduced pressure or vacuum source in the console draws or aspirates the emulsified tissue from the eye through the open end of the cutting tip, the cutting tip and horn bores and the aspiration line, and into a collection device. The aspiration of emulsified tissue is aided by a saline solution or other fluid that is injected into the surgical site through the small annular gap between the inside surface of the irrigating sleeve and the cutting tip.
One known surgical technique is to make the incision into the anterior chamber of the eye as small as possible in order to reduce the risk of induced post operative corneal curvature changes (astigmatism). These small incisions result in very tight wounds that squeeze the irrigating sleeve tightly against the vibrating tip. Friction between the irrigating sleeve and the vibrating tip generates heat. The risk of the tip overheating and burning tissue is reduced by the cooling effect of the aspirated fluid flowing inside the tip.
When the tip becomes occluded or clogged with emulsified tissue, the aspiration flow can be reduced or eliminated, which allows the tip to heat up. This practice also reduces cooling and results in a temperature increase, which may burn the tissue at the incision if left unchecked. In addition, during occlusion, a larger vacuum can build up in the aspiration tubing so that when the occlusion eventually breaks, a larger amount of fluid can be quickly suctioned from the eye, possibly resulting in the globe collapsing or other damage to the eye. Thus, it is important to dissipate the heat buildup at the incision to avoid tissue damage, and to prevent undesirable fluid surges from the eye during occlusion breaks.
Various heat generation reduction techniques are known. One way to reduce the amount of generated heat is to lessen the friction coefficient of the material that the vibrating phacoemulsification needle contacts. For instance, instead of allowing the needle to touch the rather sticky infusion sleeve made of liquid injection molded silicone, an intervening tubing made from a lower friction material such as polyimide may be employed to significantly reduce the amount of heat generated by friction. Another way is to divert irrigation flow though a bypass opening in the phacoemulsification needle in the event that the tip port of the needle becomes occluded by lens fragments. That way, irrigation flow continues to cool the needle despite the occlusion.
When the main tip port is not occluded, there will be virtually no difference in the through flow due to the presence of the bypass port, but typically clinically significant heating will occur when the main port is occluded by the lens fragments or viscoelastic material. In these cases, the presence of the bypass port can make a very significant difference by increasing flow from virtually zero to as much as 10 or perhaps more cc/min. That will result in an increase in cooling by a factor of 2-3, or perhaps even more, depending on many other factors, like the size of the sleeve used. The bypass port provides for accessory aspiration far away from the primary aspirating tip port at the distal end of the phacoemulsification needle. The bypass port is used to stabilize the anterior chamber during phacoemulsification when the primary aspirating tip is occluded. Reduction in heat generation may also be realized by lowering the vibration amplitude and/or reducing the operating duty cycle of the phacoemulsification tip.
The ultrasonically driven hand piece preferably provides torsional movement of the phacoemulsification tip. Torsional movement involves a twisting and preferably rotating movement of the tip about the longitudinal axis of the tip. Such torsional movement may be accomplished by the ultrasonic hand piece having a programmable ultrasound driver capable of producing both a torsional frequency drive signal and a longitudinal frequency drive signal. Such hand pieces are well-known to those in the art, with one example being described in U.S. Pat. No. 6,028,387 at column 2, line 6-67, column 3, lines 1-67 and FIGS. 2-3, such disclosure being incorporated herein by reference.
A conventional control system suitable for driving a torsional ultrasound hand piece may contain a drive circuit and preferably is similar to that described in U.S. Pat. No. 5,431,664, the entire contents of which being incorporated herein by reference, in that a drive circuit tracks the admittance of the hand piece and controls the frequency of hand piece to maintain a constant admittance.
The drive circuit monitors both the torsional mode and the longitudinal mode and controls these modes in the hand piece using two different drive frequencies. The torsional drive signal is approximately 32 kHz and the longitudinal drive signal is 44 kHz, but these frequencies will change depending upon the piezoelectric elements used and the size and shape of a horn. Although both the longitudinal or the torsional drive signal may be supplied in a continuous manner, preferably the longitudinal drive signal and the torsional drive signal are alternated. Such alternation enables the drive signal to be provided in a desired pulse at one frequency and then switched to the other frequency for a similar pulse, with no overlap between the two frequencies and no gap or pause in the drive signal. Alternatively, the drive signal can be operated in a similar manner as described, but short pauses or gaps in the drive signal can be introduced. In addition, the amplitude of the drive signal can be modulated and set independently for each frequency.
In the situation where chatter (visible vibration of lens fragments at the cutting tips) is present, high frequency movement of the vibrating lens or lens fragments is visibly apparent when viewed under the surgical microscope. When the lens or lens fragment vibrates less, the chatter is reduced. A softer lens will tend to chatter less, while a harder lens will tend to chatter more. Similarly, smaller fragments will tend to chatter more.
The extent that a lens is dense may be clinically estimated on a scale of 1-4 with 1 being non-compact or soft and 3-4 being dense or hard, but definitions may vary with the observer. For instance, what surgeons consider to be a hard lens in a developed part of the world will be softer than what surgeons consider to be a hard lens in the developing countries.
In the case of traditional ultrasound (longitudinal movement) along the tip axis, the lens fragments tend to be moved toward and away from the tip to give rise to chatter. In the case of torsional ultrasound (twisting movement) about the tip axis, the lens fragments move perpendicular to the tip axis and the chatter, if any, is much less than that present with traditional ultrasound for the same vibrational speed of oscillation. Nevertheless, on occasion when carrying out torsional ultrasound, chatter can still be observed when the tip is applied to very dense lenses while using a resonant frequency of 32 kHz. In addition, it is well known that lower resonant frequencies produce greater chatter.
It would be desirable to reduce or eliminate such chatter when employing torsional ultrasound on very dense or hard lenses.